NEW YORK  Even as President Bush accepted the Republican nomination Thursday and the final chapter of the campaign began, strategists in both camps were preparing for the possibility of an unprecedented situation when it ends.

An Electoral College tie.

Shifts in electoral votes and the realities of an evenly divided nation mean there is a credible case that the final tally in Bush vs. Kerry could be 269-269 — an outcome that would throw the election to the House of Representatives.

It's also easy to calculate how Bush could lose the popular vote but win the Electoral College, as he did in 2000. Or the reverse: how John Kerry could lose the popular vote but win the Electoral College.

The reaction among Americans — many of whom have forgotten the arcane details of the Electoral College that they once learned in civics class — could make the firestorm after the last election seem like a backyard barbecue.

"One of the reasons that there's so much hatred in American politics right now is the dispute over the 2000 election," says Paul Sracic, a political scientist at Youngstown State University in Ohio, one of the battleground states.

"If we had a second election like that, the passions and the hatred in this country would build even more," Sracic says. "What I fear is that the American people really don't understand these procedures. It may seem like a coup."

It's possible, of course, that the election on Nov. 2 won't be close; it usually isn't when a president runs for re-election. But if it is, officials in both parties warn that another disputed election would make the task of governing treacherous for whomever takes office on Jan. 20.

In the event of a tie

The Constitution has a process to deal with a tie in the Electoral College, which has happened once, in 1800:

 Newly elected House of Representatives would meet on Jan. 6, 2005, to choose a president from among the three top finishers. Each state delegation gets one vote.

 Newly elected Senate would choose vice president. Each senator gets one vote.

And a divided outcome — in which one candidate wins the popular vote but not the Electoral College — surely would fuel support for amending the Constitution to change the nation's election system. Proposals are pending to allocate a state's electoral votes among the candidates proportionally, or to move toward the popular election of the president altogether.

"We've never had back-to-back elections" where the results were divided, says G. Terry Madonna, director of the Center for Politics and Public Affairs at Franklin & Marshall College in Lancaster, Pa., another battleground state. "If that happens, there will be a massive movement to change the Electoral College in this country, to dump it."

Divided government?

The Constitution outlines what follows in case of a tie, which has happened only once, in 1800. The newly elected House of Representatives chooses the president from the top three finishers; each state has one vote. The newly elected Senate chooses the vice president; each senator has a vote.

(In 1800, running mates Thomas Jefferson and Aaron Burr each received 73 electoral votes, and the House gave Jefferson the presidency. Four years later, the 12th Amendment was ratified; it allows parties to nominate tickets rather than have candidates for president and vice president in effect compete with each other.)

This time, the process presumably would favor Bush. Republicans control 30 of the 50 state delegations in the House; the GOP almost certainly will keep control in the November elections. Republicans now have 51 Senate seats. But if Democrats regain an edge in the Senate — which is conceivable — the choice for vice president could get interesting.

A George W. Bush-John Edwards administration?

Republicans and Democrats are enlisting litigators, running training sessions, researching state laws and organizing SWAT teams of lawyers to respond to any problems on Election Day. Targeted are 28,000 precincts in 17 states that are expected to be close.

Democrats are paying particular attention to inner-city districts where allegations of voter intimidation arose in 2000.

Republican National Committee Chairman Ed Gillespie, noting that he proposed that the two parties create joint teams to monitor the election, expresses dismay that the Democrats haven't accepted.

But Bob Bauer, general counsel for the Democratic National Committee, calls the bid "a public-relations ploy." He says it would be "intolerable that we would rely on Republicans, with their partisan history, to help us enfranchise voters on Election Day."

Both sides say they will move more quickly than ever before to challenge results in states where the margin is narrow.

With that standard four years ago, there could have been recounts in states beyond Florida. In five states, the gap between the winner and the loser was less than one-half of 1% of the votes cast. Florida was famously determined by 537 votes. New Mexico's margin was 366 votes. Iowa was decided by 4,144, Wisconsin by 5,708 and Oregon by 6,765.

Disputes over provisional ballots — those cast by voters whose registration status is in question — could delay the results this time, Bush campaign chairman Marc Racicot says. So could the skyrocketing number of absentee ballots. Not to mention the use of election machines purchased after the brouhaha in 2000 and in some cases being used for the first time.

•For a tie: Every state votes the way it did four years ago, except for two. New Hampshire and West Virginia, which voted for Bush last time, go Democratic this time. Kerry is competitive in both states.

•For a divided result that elects Bush: Every state votes the way it did four years ago, giving Bush an electoral-vote majority of 278. (That's a more comfortable edge than last time, a side effect of the redistribution of congressional seats and electoral votes after the 2000 Census. In 2000, Bush got 271 votes, one more than required.) But Kerry carries the popular vote, as Al Gore did, by rolling up big totals in such strongholds as California and New York.

•For a divided result that elects Kerry: Every state votes the way it did four years ago, except for one. Kerry wins Florida, for a majority of 287 electoral votes, or Ohio, for 280. They went Republican in 2000; state polls released Sunday show Kerry and Bush tied in both. But Bush carries the popular vote by scoring oversized margins in his home state of Texas and in the South and Mountain West.

Before 2000, it had been more than a century since a presidential candidate won in the Electoral College without also winning the popular vote. Consultants who had spent a lifetime in politics considered the chances of it happening about as likely as, say, the Detroit Pistons upsetting the Los Angeles Lakers for the NBA championship.

But the stranger-than-fiction results four years ago — Florida's margin prevailing after a 5-4 Supreme Court ruling stopped a recount — have made the extraordinary seem plausible.

"I think we may be headed toward a nightmare scenario," says Joe Trippi, campaign manager for Howard Dean's presidential bid in the Democratic primaries. He figures Kerry could win the popular vote by a million votes — Gore won by 500,000 — while Bush carries the Electoral College.

Even mathematicians are calculating the odds. At the request of Sracic, mathematics professor Nathan Ritchey set up a Microsoft Excel program to calculate the chances for an Electoral College tie, assuming 17 battleground states.

He made adjustments last week to take into account the possibility that Maine's electoral votes could be divided between the two candidates. Maine and Nebraska allocate some electoral votes by congressional district; the other 48 states are winner-take-all.

His conclusion: There is a 1.4% chance of a tie in the Electoral College. Which happens to be the same odds of the results that occurred in 2000.

Though both campaigns are bracing for that possibility, history suggests that their preparations may not turn out to be necessary. No presidential re-election contest in modern times has been narrowly won or lost.

Presidents Dwight Eisenhower, Richard Nixon, Ronald Reagan and Bill Clinton won re-election decisively; Jimmy Carter and the elder George Bush bombed. Polls showed some of those contests nearly even until the final weeks.

"Since these elections often turn out to be referenda on the incumbent president, undecided voters seem to make a collective decision in the final few days about whether the president deserves to be re-elected," says Stuart Rothenberg, editor of the non-partisan Rothenberg Political Report. "That can swing the results rather dramatically, providing either a decisive defeat or a clear victory."

On the other hand, no presidential contender has managed to get a majority of the popular vote since 1988. President Clinton was elected by pluralities, the younger President Bush by less than that.

Remembering Indiana

Both campaigns are ready to apply lessons learned from past recount battles.

The battle in Florida four years ago was hardly the first. Republicans cite the showdown over the 8th Congressional District of Indiana in the 1984 election with the kind of emotion that veterans of other battles remembered the Alamo or the sinking of the Maine.

In that contest, state officials certified Republican Richard McIntyre as the winner by 34 votes. But the Democratic-controlled U.S. House, after recounts, declared Democrat Frank McCloskey the winner by four votes — the narrowest margin in a congressional race in U.S. history.

After Indiana, Republicans adopted more aggressive tactics, according to Rich Galen, a GOP consultant who was then working for the National Republican Congressional Committee. And they have paid as much attention to the public relations of a recount as its legalities. Both points proved helpful in the presidential recount in Florida four years ago, he says.

Democrats cite lessons learned in Tallahassee, too. Marc Elias, general counsel for the Kerry campaign, has organized thousands of volunteer lawyers across the country. At the Democratic National Committee, Bauer has put together teams of election experts, briefcases at the ready.

"We will be able to handle a multifront war," Elias says, with enough teams in place to handle challenges in up to five states.

Republicans are drawing up their own blueprints, including sessions for their corps of volunteer lawyers. "We're working to train lawyers to make sure they understand the rules," especially new election laws, Bush campaign manager Ken Mehlman says.

What may become the newest election law of all: Colorado voters in November will vote on a measure that would allocate the state's nine electoral votes proportionately to the popular vote, starting with this election. That means each major-party candidate almost certainly would get some of them.

Which would, mathematically speaking, make it even easier to reach a tie.